In the approximation, \(a\)  is monotonically decreasing at a steady rate of \(2b^3\kappa ^3\)  per step.
The one step update of \(b\)  is flipping signs and contains second and third order terms of \(b\) . For the two-step
approximation however, the oscillation behavior and the even-order terms of \(b\)  all cancels. This is consistent with the analysis in [1]} that the two step dynamics travels along a sharpness reducing flow.
